Since lithium ion secondary batteries were commercialized by Sony Corp., Japan, in the year 1991, they have been widely used as power sources of portable devices. With the recent development of the electronic, communication and computer industries, camcorders, mobile phones, notebook PCs, etc., have appeared and advanced remarkably, and the demand for lithium ion secondary batteries as power sources for driving such portable electronic information communication devices has increased day by day. Since hybrid electric vehicles (HEVs), which use internal combustion engines and nickel-hydrogen batteries as power sources, were put to practical use by Toyota Motor Co., Japan, in the year 1997, studies thereon have been actively conducted. Recently, studies using, as power sources for hybrid electric vehicles, lithium secondary batteries having excellent high energy density and excellent low-temperature operational characteristics compared to those of nickel-hydrogen batteries, have been actively conducted. In order to commercialize the lithium secondary batteries for hybrid electric vehicles, a cathode (positive electrode) material, comprising about 35% of the price of battery materials, must be an inexpensive material. The most suitable material for HEVs is spinel LiMn2O4, which operates at a potential of 4V and has low-cost and high-output characteristics. However, this spinel LiMn2O4 shows a reduction in capacity with the passage of time, because manganese therein is dissolved in an electrolyte as charge/discharge cycles progress. Particularly, this phenomenon is accelerated at a high temperature of more than 55° C., and thus it is impossible to use the spinel material in actual batteries. In an attempt to solve this phenomenon, many studies focused either on substituting the manganese site with lithium and transition metals or substituting the oxygen site with fluorine have been conducted (J. of Power Sources, 101(2001), 79). Among them, the most effective is Li1+x Niy Mn2-x-yO4-zFz (0.04≦x≦0.06, 0.025≦y≦0.05, 0.01≦z≦0.05), in which the manganese site is substituted with lithium and Ni and the oxygen site is substituted with fluorine. This material is known to have the best cycle life characteristics (J. Electrochem. Soc. 148 (2001) A171). However, this material also shows a slow decrease in capacity at high temperatures. In another attempt to solve this problem, studies on coating the surface of the spinel cathode material with ZnO in order to neutralize fluoric acid in an electrolyte and protect the surface have been conducted. However, this method did not completely solve the problem of the decrease in capacity caused by the dissolution of manganese.
Currently commercially available small-sized lithium ion secondary batteries employ LiCoO2 in the cathode, and carbon in the anode. LiCoO2 is an excellent material having stable charge/discharge characteristics, excellent electronic conductivity, high stability and smooth discharge voltage characteristics, but cobalt occurs only in small deposits, is expensive, and is, furthermore, toxic to the human body. For this reason, other materials need to be studied. Cathode materials, which are actively studied and developed, may include LiNi0.8Co0.2O2, LiNi0.8Co0.1Mn0.1O2, and Li[NixCo1-2xMnx]O2, such as LiNi1/3Co1/3Mn1/3O2, and LiNi1/2Mn1/2O2. LiNi0.8Co0.2O2 or LiNi0.8Co0.1Mn0.1O2 having a layered structure, like LiCoO2, has not yet been commercialized, because they are difficult to synthesize at stoichiometric ratios and have problems related to thermal stability. However, a core-shell cathode active material, which uses LiNi0.8Co0.2O2 or LiNi0.8Co0.1Mn0.1O2, having a high capacity, in the core, and a layered transition metal LiNi1/2Mn1/2O2, having excellent thermal stability and cycle-life characteristics, in the shell, was recently synthesized (Korean Patent Application No. 10-2004-0118280). This material has significantly improved thermal stability and cycle-life characteristics. Recently, LiNi1/3Co1/3Mn1/3O2, a kind of Li[NixCo1-2xMnx]O2, has started to be substituted for LiCoO2, and the use thereof as a cathode material for small-sized batteries, particularly by Japanese battery makers Sanyo Corp. and Sony Corp. has increased. Recently, LiMn2O4, which is environmentally friendly and inexpensive, has been used in some of small-sized batteries, but is expected to be used mainly in large-sized batteries, particularly hybrid electric vehicles (HEVs). However, 4V-grade spinel LiMn2O4 has poor cycle-life characteristics, because of structural transition called Jahn-Teller distortion, which is caused by Mn3+, and the resulting Mn dissolution. Particularly, at a high temperature of more than 55° C., the manganese dissolution in this material is accelerated by reaction with fluoric acid (HF) in an electrolyte to rapidly deteriorate the cycle life characteristics of this material, making it difficult to commercialize this material.
Recently, a patent was reported in which was prepared a double-layer cathode material consisting of a core made of 4V-grade spinel LiMn2O4 and a shell made of LiNi0.5Mn1.5O4, causing no manganese dissolution problem, using a carbonate method (Korean Patent Application No. 10-2005-0027683). The cathode material prepared using the carbonate method doesn't have significantly improved cycle life characteristics, because it has a large specific surface area, but it is difficult to control the composition thereof. Also, manganese oxide prepared using a coprecipitation method with the carbonate method has low tap density, which makes it difficult to obtain batteries having high energy density. Furthermore, because the manganese oxide consists of secondary particles having weak binding force between primary particles, it is difficult to maintain the manganese oxide in a spherical particle shape upon the production of battery electrode plates.